Bleeding after cardiopulmonary bypass (CPB) is a serious complication of cardiac surgery and is associated with substantial morbidity and mortality; neonates in particular are vulnerable to post-CPB bleeding. In these patients, transfusion post-CPB is a necessity. Transfused blood products consist primarily of platelets and fibrinogen (in the form of cryoprecipitate) prepared from adult blood. Although the transfusion of adult blood products to pediatric patients is performed routinely to control blood loss, reduction in the usage of transfused blood products has been shown to correlate with better patient outcomes. Unfortunately, few alternatives exist, highlighting the need for better methods to control bleeding. Hemostasis involves the formation of a platelet plug embedded within a fibrin mesh. However, clot formation is impaired in neonates, in part, due to deficiencies in several coagulation factors at birth and platelet hyporeactivity. CPB further impairs these already compromised platelets. Thus, the overarching design goal of this project is to create a biomaterial that recapitulates key hemostatic functions of platelets to augment clotting in neonatal CPB patients. To achieve this objective, we utilize platelet-like particles (PLPs), recently designed in our lab, that mimic numerous features of natural platelets by interacting with fibrin with high affinity and, more importantly, specificity at the sites of injury to augment clotting in plasma samples obtained from neonates undergoing CPB. To achieve this objective, the proposal is divided into two specific aims: 1. Characterization of platelet function and clottig in neonatal CPB plasma samples in the a) absence and b) presence of PLPs and 2. Characterization of augmentation of clotting in vivo in a rodent model of platelet hyporeactivity. In Aim 1, we will analyze neonatal platelet function in blood samples collected prior to CPB (baseline), post-CPB and post-transfusion from neonates undergoing elective cardiac surgery, through thromboelastography/platelet mapping assays and dynamic clotting assays utilizing a novel endothelialized microfluidic device developed in our labs. We will then perform identical analyses with post-CPB samples in the presences of PLPs. Because neonatal patients are known to have hyporeactive platelets, in Aim 2, we will model this condition and evaluate the efficacy of PLPs in vivo in a small animal platelet deficiency model by monitoring bleeding times. Furthermore, the biodistribution and circulation half-life of PLPs will be evaluated. The proposed research is innovative because it combines unique microgels with fibrin specific binding motifs to create PLPs that interact extensively with fibrin networks. These features result in particles that are capable of recapitulating more features of nature platelets than previously achieved by other platelet-mimicking materials. The significance of the proposed research is that this work will enable better treatment options for coagulopathy in neonatal CPB patients.